It is known that, when two Al—Zn—Mg series alloy members are welded, the welded portion is re-solution heat treated by the welding heat and once softened, but the strength thereof will be regained by the subsequent natural aging. Therefore, for a member that needs to be welded and also have a high strength among structural elements, an Al—Zn—Mg series alloy has been generally used.
On the other hand, a technology for producing a structural member with an aluminum alloy forged product is well known. For example, as described in Patent Document 1 listed below, a forged product is produced by forging an Al—Zn—Mg series alloy extruded member as a forging material.
Generally, when forging an Al—Zn—Mg series alloy extruded member as a forging material, the plastic deformation performance of the extruded member (forging material) at the time of the forging differs between in the extrusion direction and in a direction perpendicular to the extrusion direction, causing anisotropy of plastic deformation.